a) Field of the Invention
The present invention relates to a performance improved reinforced composite pipe and hollow member wherein the improvement comprises the application of at least one additional exterior layer of circumferential wound fiber and thermoset polymer to a cured fiber reinforced composite pipe and hollow member. The performance improvement results from the prestress tension of the circumferential winding creating compression stresses in the cured pipe and hollow member that counteract the tension stresses due to operational loadings. For the purposes of this invention the generally accepted definition of the term cryogenic is taken as the temperature range from liquid carbon dioxide at minus 80 degrees centigrade to liquid helium at minus 268.9 degrees centigrade.
Composite piping has been judged to be inadequate for general cryogenic applications due to crazing and micro cracking which are visible indications of failure.
For the purposes of this current invention the term "fiber" is defined to mean a slender and greatly elongated shape from natural and synthetic materials which has tensile strength and comprises fiber, thread, fiber bundles, filament bundles, rod, and wire.
For the purposes of this current invention the term prestress is categorized as either macro or micro. "Macro prestress" is defined as when prestress is applied to a composite by mechanical means on the component as a unit, for example by hydraulic, temperature expansion, or pneumatic means. "Micro prestress" is defined as when prestress is applied on an incremental basis to each individual fiber winding to produce a cumulative prestress affect on the component.
b) Description of the Prior Art
Prestress of a tendon, hoop, or membrane is well recognized in a broad range of applications from pre-cast concrete to textile tent structures. In the metal forming art, externally induced prestress has been used to improve the stress response of tubular sections from barrel hoops to gun barrels. However, with polymer composites, a design goal has existed to eliminate internal residual stresses during manufacture. This goal and the reasons behind it have obviated the use of prestress as a performance enhancement for hollow bodies such as tanks and pipes.
Particularly for aerospace composites, care is taken to eliminate residual stresses since this assures the full design stress being available to resist aerodynamic loadings. For polymer composite airframes the elimination of residual stresses assures that the cantilever loads induced by gravity on a plane's wings sitting in a hanger will not add internal stresses to cause long term creep warping of the aerobatic surfaces.
Another reason that prestress winding was not an acceptable tool for composites in the prior art is that prestress sufficient to balance operating loadings would create problems in further steps of the manufacturing process. For example, tensioned fiber would cut into previous layers of the uncured matrix and displace previous layers of fibers. In both aerospace and industrial polymer composites, design procedure dictates that fabrication should be complete prior to cure of the part. Means of cure such as oven baking provides cross-linking which results in an increase in the polymer strength. Substantial cumulative prestress on an uncured composite component would create yielding that would deform the part during curing. The warping of carefully fabricated unstressed composite assemblies during curing has tended to make prestress abhorent.
In the prior art when prestress has been applied to cured composites, it has been accomplished by mechanical means of "macro prestress". Sigur, U.S. Pat. No. 5,084,219, uses thermal expansion of a mandrel to compress previous layers of a composite material into the fiber mesh. The fiber is prestressed under tension. Sigur uses this "macro pre-stress" during manufacture, but the result is not directly related to predictable composite performance. Piramoon, U.S. Pat. No. 5,057,071, accomplishes this "macro prestress" by alternately heating and cooling components prior to assembly. Walls, et. al., U.S. Pat. No. 4,996,016, accomplish this "macro prestress" by the use of a bonding agent under pressure applied to components immediately prior to assembly.
Still another reason the utility of auxiliary prestress has been ignored and avoided for polymer composites has been the comparatively small capacity of some high modulus fibers to resist compressive loads. The cross section of a typical pipe under internal pressure shows that the compression is not axial to the fiber but transverse to the fiber. A patent where circumferential winding is used for fabrication, Fawley, U.S. Pat. No. 5,289,942, carefully uses "insubstantial" to describe the amount of tension or "micro prestress" in the winding. Fawley applies filaments with an "insubstantial" tension to a storage tank, using only enough tension to permit them to adhere to the storage tank and to avoid displacing prior layers of filaments.
"Substantial" prestress does exist in the prior art in Rose, U.S. Pat. No. 5,429,693. Rose uses "micro pre-stress" in the manufacture of a hollow body composite to balance the elongation between fiber and polymer for each layer of a fiber reinforced hollow body using induced strain. Rose is a system for reducing the resultant residual tension stresses in the polymer matrix by nearly half after cure. Rose inflicts "substantial" tension in the fiber (106,000-112,000 psi.). However, Rose induces tension in the fiber both by mechanical means and by expansion of the aluminum mandrel when raised to the curing temperature in order to eliminate manufacturing defects. Rose does not use this prestress to predictably extend hollow body performance under operating loads.
Rose discloses a method of making a prestressed composite material in which prestress is used to balance out inherent stress differences between the polymer and the fiber during construction of the uncured composite material. Although a "substantial" prestress tension is described, it is substantially below that required to overcome tensions caused by operational loadings. This combined micro and macro prestress described by Rose is limited by the ability of an uncured partly assembled composite material to carry the prestress to the mandrel on which it is mounted.
"Substantial" prestress also is taught by Bettinger, U.S. Pat. No. 5,552,197. For cylindrical sections Bettinger uses prestress in the manufacture of pipe connectors that release substantially all their prestress when heated to clamp together pipe members. Bettinger uses this "micro prestress" for assembly, not to predictably extend pipe performance under extreme conditions.
Bettinger discloses composite connectors which generate delayed dimensional change and force due to prestressed fibers constrained and controlled within and by a responsive polymer matrix. Bettinger does not counteract the tension component of the stresses generated under operating conditions that would cause deformation and cracking since most or all of the "micro prestress" is used up in decreasing the diameter during activation.
In the prior art of "micro prestress" Swartout, U.S. Pat. No. 4,370,899, creates a prestress state in a fiber composite material, solid core flywheel rim by winding the fibers with high tensile stress. However, the 50,000 psi. maximum tension winding stress of Swartout is insufficient to counterbalance the tension stresses due to structural, as well as hydraulic, pneumatic, and temperature operational loadings and to improve composite performance under extreme conditions.